Another thing worth mentioning is that you can manipulate the second transform to create gaps in the fractal and then add a third transform with some variation of your own choice. This third transform will actually cause a secondary fractal to become nested in those gaps, and adds a lot more detail to your fractal.

nice tutorial, easy to follow. A small update for users of updated Apophysis version. Symmetry is called color speed now, and you'll need to add flatten to the first transform variation, or it'll actually get darker as the density increases.